Question: $ D = \left[\begin{array}{rrr}2 & 1 & 2 \\ 0 & 2 & 1 \\ 2 & 0 & 0\end{array}\right]$ What is $ D^{-1}$ ?
Answer: $ = \left[\begin{array}{rrr}0 & 0 & \frac{1}{2} \\ -\frac{1}{3} & \frac{2}{3} & \frac{1}{3} \\ \frac{2}{3} & -\frac{1}{3} & -\frac{2}{3}\end{array}\right]$